A SUPER FAST, (SUPER LOUD) MINISUB

29 Jul 2009

The Day (New London, CT) on Monday had an intriguing article about DARPA's Underwater Express. This program aims to prove engineering approaches for a manned minisub able to carry high value cargoes submerged at 100 knots -- a "super-fast submerged transport," or SST. Underwater Express was announced with a request for proposals in 2005. The RFP specified supercavitation, a form of enhanced submerged propulsion exploiting a self-made vacuum cavity or gas envelope between hull and ocean to reduce flow resistance by "60 - 70%." Supercavitation, such as used in the Soviet-Russian Shkval rocket torpedo, is extremely noisy. Even allowing for a breakthrough in how the gas cavity is created and maintained, the classic power-versus-speed formula makes it highly likely that only a rocket engine could achieve the required 100-knot speed for the SST. Yet the RFP mentioned nothing about silencing the technology demonstrator minisub.

After a competition, General Dynamics Electric Boat was awarded a contract which by completion is expected to total $38 million. The deliverable will be a quarter-scale unmanned version of its winning design, to be demonstrated in the waters off New England in spring 2010. The demo is to include runs at up to 100 knots for 10 minutes, with maneuvers to show that the SST is safe at such speeds. GDEB says they've solved the challenges of maintaining a stable gas envelope while accurately controlling the test vessel's depth, course, angle of attack, and speed. Details are top secret.

I'd been wondering what good there might be to a manned minisub that, unlike a rocket torpedo, has to be reusable and survivable -- but which would, whenever moving fast, make a huge passive sonar signature, broadcasting its presence to any enemies for miles around. Besides, what missions would it be used for that couldn't be done by a HALO insertion and Osprey extraction, or for that matter by a slow moving battery-powered mini like some Improved ASDS? When The Day's article came out, I decided to ask a source. The rest of this is my interpretation of the answers I got, sprinkled with public info and my own conjectures and commentary.

Underwater Express might for instance be used to extract a SEAL team from a beach, perhaps along with an important defector or prisoner, or maybe with some "confiscated" nuke warhead cores or other nuclear tech, in environments where air control is disputed by cheap but lethal man-portable missiles, but local undersea surveillance is weak due to the local regime's technical limitations plus coastal environmental noise. Such missions need not be planned for actual wartime. They could be used instead for smash-and-grab police actions against rogue states or terrorists threatening to use WMDs. They might be used to rescue kidnap victims, hostages, or POWs. The key to an SST's possible utility is that it provides the option to go in and out fast but noisy, rather that quiet but slow.

The tactical advantage of going in and out real fast, underwater, would be the plausible deniability of the whole operation afterward, combined with the rapidity -- barely 7 minutes each way -- at which the mini and its commandos and passengers/cargoes could dart inshore and away again from the relative sanctuary of international waters beyond the 12-mile limit. (Presumably, the U.S. could attribute any sonar recordings of the SST op, flogged to CNN or Al Jazeera, as "just the usual lava displacements.") Total surprise, and speed and precision of mission execution, would obviously be essential, but Navy SEALs and other elite special ops folks excel at exactly such traits.

The mini could be quietly dropped off by a full-size nuclear sub that stealthily leaves the area, before the mini starts the mission clock ticking by firing up its main engine(s). It might be picked up right after the mission, quickly and somewhat covertly, via one of the flooded amphibious warfare docks available within a passing surface-ship strike group, with SSN escort, that would certainly provide a powerful 3-D self-defense envelope. This strike group might be deployed to do broad racetracks off the hot-spot coast from the moment when tensions first start to heat up toward a crisis point, keeping the enemy guessing as to what sort of raid(s) might be attempted, where, when. In fact, the strike group might conduct repeated LCAC exercises outside territorial waters, before and after the undersea raid. Rather deafening themselves when zooming along on their air cushions about as fast as a fleeing SST would go, they could lull hostile defenses in advance, and serve as very handy diversions and mobile noisemakers when the time came.

While the noise of a supercavitating SST would make acoustic detection likely, even without LCAC support its corresponding high speed and ability to zig-zag could render the rest of the opposition force's targeting cycle -- localization, identification, tracking, and accurate attack -- extremely difficult. With a sustained top speed of 100 knots, the mini could outrun any conventional torpedo; due to its speed and maneuverability, it would be very hard to hit with even a supercavitating anti-submarine weapon. It could be there and gone before the enemy has any chance to start to react, say by launching some helos with active sonobuoys, or vectoring in patrol boats with depth charges.

At least that seems to be the idea. The Day's piece points out that controllable, maneuverable supercavitation might yield big dividends in more efficient propulsion systems for civilian ocean transportation. Remember, Underwater Express is a proof of technology project only. The current EB contract is not meant to produce an operational SST.